I passed by the Messenger of Allah ﷺ while he was praying and greeted him. He returned my greeting with a gesture.The narrator said, "I only know that he said it was a gesture with his finger."
حدثنا قتيبة حدثنا الليث بن سعد عن بكير بن عبد الله بن الأشج عن نابل صاحب العباء عن ابن عمر
مررت برسول الله ﷺ وهو يصلي فسلمت عليه فرد إلي إشارة وقال لا أعلم إلا أنه قال إشارة بإصبعه
[Sahih]. Bukhari also reported it in the Book of Prayer, chapter on returning the greeting during prayer, hadith no. 925, and Ibn Khuzayma graded it sahih, hadith no. 888, as did Ibn Hibban. Nasai reported it, 5/3, hadith no. 1187, from Qutayba with the same wording (al-Ihsan), 2255, as did Ibn Jarud, hadith no. 216, and Hakim, 12/3, and Dhahabi, from another route. Ibn Majah reported it, hadith no. 1017.The hadith has corroborating narrations. On this topic there are also narrations from Bilal [to come, no. 368], Abu Hurairah [Abu Dawud, hadith no. 944], Anas [Abu Dawud, hadith no. 943], and Ayesha [Bukhari, hadith no. 688, and Muslim, hadith no. 412, in which it states, "So he gestured to them to sit down."].