]وَعَنْ عِمْرَانَ بْنِ حُصَيْنٍ ‏-رَضِيَ اَللَّهُ عَنْهُمَا‏- قَالَ : { قَالَ لِيَ اَلنَّبِيُّ ‏- صلى الله عليه وسلم ‏-" صَلِّ قَائِمًا, فَإِنْ لَمْ تَسْتَطِعْ فَقَاعِدًا, فَإِنْ لَمْ تَسْتَطِعْ فَعَلَى جَنْبٍ } رَوَاهُ اَلْبُخَارِيُّ.‏ 1‏

‏1 ‏- صحيح.‏ رواه البخاري (1117)‏.‏

Translation
Narrated Ibn 'Umar (RA)

I asked Bilal (RA), "How did you observe the Prophet (ﷺ) replying to their (the Companions) salutation while he was engaged in prayer?" He (Bilal) said: "He used to do this way," and he (Bilal) demonstrated by spreading his palm.[Reported by Abu Dawud and at-Tirmidhi. The latter graded it Sahih (authentic)].