Sahih Ibn Khuzaymah

Hazrat Azraq bin Qais narrates that he saw Sayyiduna Abu Barzah Aslami (may Allah be pleased with him) performing prayer while holding the reins of his mount in his hand. When he bowed for ruku‘, the reins slipped from his hand and the mount started to move. So Sayyiduna Abu Barzah (may Allah be pleased with him) walked backwards, without turning his attention here or there, until he caught up with his mount and took hold of it. Then, in that same state, he walked back to his place of prayer and completed his prayer, ending with salam. Then he said: Indeed, I have participated in many battles with the Messenger of Allah (peace and blessings be upon him). He counted many battles and said: I have seen the concessions and eases of the Prophet (peace and blessings be upon him), and this concession (walking during prayer when needed) I have taken from among them. And if I had left my mount like that until it reached the wilderness, and then I, being an old man, had to search for it in the darkness of the night, it would have been very difficult and burdensome for me.