The Prophet ﷺ said, "When dawn breaks, the time for all night prayer and witr has passed, so perform witr before dawn breaks."
عن النبي ﷺ قال: إذا طلع الفجر، فقد ذهب كل صلاة الليل والوتر، فأوتروا قبل طلوع الفجر.
Tirmidhi (469) narrated it from Mahmud bin Ghaylan, who said, "Abdul-Razzaq narrated to us, Ibn Jurayj informed us, from Sulayman bin Musa, from Nafi, from Ibn Umar," and he related it.Nawawi said in Khulasa (1906), "Tirmidhi narrated it with a sahih chain." Tirmidhi said, "Sulayman bin Musa alone narrated it with this wording, and it has been narrated from the Prophet ﷺ, 'There is no witr after the morning prayer.'"I say, Sulayman bin Musa is the freedman of the Umayyads, the Damascene, known as Ashdaq. Darimi and Ibn Saad regarded him as trustworthy, while Bukhari and Nasai criticized him. The summary of his status, as given in Taqrib, is "truthful, a jurist, with some weakness in his hadith, and he became confused shortly before his death." So Nawawi's declaring this sahih is questionable, and in most cases its status is hasan because of Sulayman bin Musa. Perhaps it was due to his confusion that he narrated it once as raised (marfu) and another time as stopped (mawquf).Hakim (1/302) and Bayhaqi (2/478) both narrated it via Hajjaj bin Muhammad, who said, "Ibn Jurayj said, 'Sulayman bin Musa informed me, Nafi narrated to us, that Ibn Umar used to say, "Whoever prays at night should make the last of his prayer witr, for the Messenger of Allah ﷺ commanded that. When Fajr comes, the night prayer and witr are gone, because the Messenger of Allah ﷺ said, 'Perform witr before Fajr.'"'"Hakim said, "The chain is sahih."I say, it is not as he claimed. Although Ibn Jurayj clearly stated that he was informed directly, the defect lies with Sulayman bin Musa. Either the matter became confused for him, or Ibn Umar himself narrated it in this way, sometimes as a raised report and sometimes as his own stopped statement derived from something the Prophet ﷺ had said, which would then agree with what Muslim narrated, "Hasten to perform witr before dawn."As for Tirmidhi's statement, "It has been narrated from the Prophet ﷺ, 'There is no witr after the morning prayer,'" this is extremely daif.Ibn Nasr narrated it in Kitab Witr (69), and Abdul-Razzaq (4591), both via Abu Harun Abdi, from Abu Saeed Khudri, who said, "The Messenger of Allah's ﷺ announcer called out, 'There is no witr after Fajr.'" Abu Harun Abdi is Umara bin Juwayn. The scholars of hadith do not rely on his narrations, he is extremely daif, and he has been accused of lying.Ibn Nasr said, "This is a hadith that, if established, would be conclusive evidence that could not be contradicted, except that the scholars of hadith do not rely on the narration of Abu Harun Abdi."And that is indeed the case, for Abu Harun Abdi is extremely daif.But it is established from the hadith of Abu Saeed Khudri that "whoever is overtaken by dawn without having performed witr has no witr," and this hadith is sahih. So this should be understood as referring to one who deliberately delays witr until dawn overtakes him.