Sufyaan [ibn ‘Uyayanah] narrated to us from Zayd ibn Aslam from ‘Abdullaah ibn ‘Umar said,
“The Messenger of Allah, peace be upon him, entered the mosque of Banu ‘Amr ibn ‘ Awf, Masjid Qubaa’, to pray. Some men from among the Ansaar (Helpers) came in to greet him. Suhayb was with him. So I asked Suhayb, ‘What did the Messenger of Allah do when he was greeted [while in the prayer]?’ He replied, ‘He gestured with his hand.”‘ Sufyaan said, “I said to a man: Ask Zayd [ibn Aslam] if he had heard this report from (Abdullaah [ibn (Umar], for I felt too shy to ask him myself. The man asked [Zayd], ‘ 0 Abu Usaamah , did you hear it from (Abdullaah ibn (Umar?’ Zayd replied, ‘Yes, I saw him and I even spoke to him.”‘
Musnad Ahmad, Vol. 2, p. 10. The isnaad of this Hadeeth is saheeh according to the conditions set by al-Bukhaaree and Muslim.
Commentary
It was the Prophet’s practice that whenever he was greeted while he was in prayer he would sometimes gesture with his right hand by moving it a little away from the left with the palm facing down. At other times, he would gesture with his index finger, and yet at other times, he would move his head. All of these ways of responding to greetings have been related by several companions, such as Abdullah ibn Umar, Bilaal ibn Rabaah and Jaabir ibn Abdullah (may Allah be pleased with them). We understand from this that there will be no blame on anyone who enters the mosque and greets the Muslims even if they are engaged in prayer, for he, (peace be upon him) did not object to those people who greeted him while he was praying.
Source:
Thulaathiyyaat Musnad Ahmad ibn Hanbal (may Allah have mercy on him)
Reports of Imaam Ahmad that have only three narrators between him and the Prophet, peace and blessings be upon him.
Shaykh Muhammad ibn Ahmad ibn Saalim as–Saffaareenee al–Hanbalee ~(1114 –1188 AH)