ENOfficial
Ibu ‘Umar said he asked Bilal how the Prophet (ﷺ) responded to them when they greeted him while he was engaged in prayer, and he replied, “He used to make a sign with his hand.” Tirmidhi transmitted it. In Nasa'i’s version there is something similar, but Suhaib occurs instead of Bilal.
Source: sunnah.com
Other translations
Arabic
وَعَنِ ابْنِ عُمَرَ قَالَ: قُلْتُ لِبِلَالٍ: كَيْفَ كَانَ النَّبِيُّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يَرُدُّ عَلَيْهِم حِين حانوا يُسَلِّمُونَ عَلَيْهِ وَهُوَ فِي الصَّلَاةِ؟ قَالَ: كَانَ يُشِيرُ بِيَدِهِ. رَوَاهُ التِّرْمِذِيُّ وَفِي رِوَايَةِ النَّسَائِيِّ نَحوه وَعوض بِلَال صُهَيْب
sunnah.com