He came upon the Prophet ﷺ while he was bowing, so he made the bowing before reaching the row. When this was mentioned to the Prophet ﷺ, he said, "May Allah increase you in eagerness, but do not do it again."
أنه انتهى إلى النبي ﷺ وهو راكع فركع قبل أن يصل إلى الصف، فذكر ذلك للنبي ﷺ فقال: زادك الله حرصا، ولا تعد.
Bukhari narrated it in the chapter on adhan (783), from Musa bin Ismail, who said, Hammam narrated to us, from Alam, who is Ziyad, from Hasan, from Abu Bakra, and he related it. Abu Dawud (684) narrated it from Musa bin Ismail, Hammad narrated to us, Ziyad Alam informed us of it, and his wording was, Abu Bakra came while the Messenger of Allah ﷺ was in ruku, so he did ruku before joining the row, then walked to the row. When the Prophet ﷺ finished his prayer, he said, "Which of you did ruku before joining the row, then walked to the row?" Abu Bakra said, "I did." The Prophet ﷺ said something similar to him.It has already been established, before this, from the action of Zayd bin Thabit, that he found the people in ruku, so he did ruku, then crept along until he reached the row. Malik narrated it in Muwatta, and Bayhaqi (2/90), and its chain is sahih.Bayhaqi narrated in his Sunan (2/90–91), through the route of Zayd bin Wahb, who said, "I went out with Abdullah bin Masud from his house to the masjid. When we reached the middle of the masjid, the imam did ruku, so Abdullah said Allahu Akbar and did ruku, and I did ruku with him. Then we walked while in ruku until we reached the row, at the moment the people raised their heads. When the imam finished the prayer, I stood up thinking that I had not caught the rakat, so Abdullah took my hand and made me sit down, then said, You have indeed caught it." Its chain is sahih.In this hadith there is evidence for the majority position, held by those who say that whoever catches the ruku has caught the rakat, namely the four imams, Uzai, Thawri, Ishaq, and others, because the Prophet ﷺ did not command Abu Bakra to make up this rakat, and it is the practice of the Lawgiver not to delay clarification past the time of need. A group of scholars held this view, and it is what His Eminence Shaykh bin Baz, may Allah have mercy on him, and others gave as their formal opinion.Similar in meaning is what was reported from Abu Hurairah as a raised hadith, "When you come to the prayer while we are in prostration, then prostrate, and do not count it as anything, and whoever catches the rakat has caught the prayer." Abu Dawud narrated it (893) from Muhammad bin Yahya bin Faris, that Saeed bin Hakam told them, Nafi bin Yazid informed us, Yahya bin Abu Sulayman told me, from Zayd bin Abu Attab and Ibn Maqburi, from Abu Hurairah, and he related the hadith.Ibn Khuzayma narrated it (1622), and Hakim (1/274, 273), and from him Bayhaqi (2/89), through the route of Yahya bin Abu Sulayman, in similar fashion. Ibn Khuzayma said, "There is something in my heart regarding this chain, for I did not know Yahya bin Abu Sulayman to have been credited with either integrity or discredit." Hakim said, Yahya bin Abu Sulayman is among the trustworthy narrators of the Egyptians, and he said elsewhere, he was from Madinah and settled in Egypt. End of quote.Hakim is known for being lenient in judging men. As for this Yahya, major critics have spoken about him. Bukhari said in Juz Qira, "This Yahya is one whose hadith is denounced, his hearing from Zayd bin Abu Attab is not established, nor from Saeed bin Abu Saeed Maqburi, and no proof can be established by him." Abu Hatim said, "His hadith is disordered." Bayhaqi said, Yahya bin Abu Sulayman Madini alone transmitted it.It has also been narrated through another chain, weaker than that, from Abu Hurairah, which is what he himself, and Daraqutni (2/346), narrated through the route of Ibn Wahb, Yahya bin Humayd informed me, from Qurra bin Abdul-Rahman, from Ibn Shihab, who said, Abu Salama bin Abdul-Rahman informed me, from Abu Hurairah, that the Messenger of Allah ﷺ said, "Whoever catches a rakat of the prayer has caught it, before the imam straightens his back."Abu Ahmad (Ibn Adi the hadith master) said, "This addition, 'before the imam straightens his back,' is said by Yahya bin Humayd, from Qurra, and he is Egyptian." He said, I heard Ibn Hammad say, Bukhari said, "Yahya bin Humayd, from Qurra, from Ibn Shihab, Ibn Wahb heard from him, an Egyptian, his hadith is not corroborated." End of what is in Sunan al-Kubra.In Mizan, Daraqutni declared him daif. As for Qurra bin Abdul-Rahman, Muslim cited him among the supporting witnesses. Juzjani said, "I heard Ahmad say, his hadith is severely denounced." Yahya said, "His hadith is daif." Abu Hatim said, "He is not strong."