As for agency in matters a man is obliged to undertake on behalf of others, examples include executors and agents entrusted with authority appointing others to act in their stead in fulfilling their duties towards those who appointed them or those under their care. Another example is the ruler appointing deputies to discharge his responsibilities concerning the Muslims. Whenever the Messenger of Allah (may Allah bless him and grant him peace) left for a military expedition, Hajj, or ‘umrah, he would appoint someone to govern Medina in his absence. He would also send his officials to the provinces and appoint commanders over the armies.
Examples include “his sending Mu‘ādh ibn Jabal and Abū Mūsā al-Ash‘arī to Yemen as teachers. When they arrived in Yemen, they lodged separately and later met. Mu‘ādh asked Abū Mūsā, ‘How do you recite the Quran?’ Mālik said: I believe they had come to teach the people Islam and the Quran. Abū Mūsā replied, ‘As for me, I recite it a little at a time—walking, riding, sitting, standing, and in every circumstance.’ Mu‘ādh said, ‘As for me, I sleep during the first part of the night and rise during its latter part, seeking reward for my sleep just as I seek reward for my standing in prayer.’”
“And his appointment of ‘Attāb ibn Asīd as governor of Mecca?”
“He (may Allah bless him and grant him peace) sent an army to Mu’tah and appointed Zayd ibn Hārithah as its commander: if Zayd were killed, Ja‘far ibn Abī Tālib would command them; and if Ja‘far were killed, ‘Abd Allāh ibn Rawāhah would command them. They set out until they encountered Ibn Abī Sīrah al-Ghassānī at Mu’tah, where forces of Arab Christians and Romans had gathered. Ibn Abī Sīrah shut the fortress against the Muslims for three days. Then they came out, the two sides met, and fierce fighting ensued. Zayd ibn Hārithah took up the standard and was killed; then Ja‘far ibn Abī Tālib took it and was killed; then ‘Abd Allāh ibn Rawāhah took it and was killed. They then agreed upon Khālid ibn al-Walīd as their commander, and Allah defeated the enemy and granted the Muslims victory.
“Before news of their deaths reached Medina, the Messenger of Allah (may Allah bless him and grant him peace) said, ‘Ja‘far ibn Abī Tālib was brought past me among the angels, flying as they fly, with two wings.’
“Ya‘lā ibn Manī‘ came to the Messenger of Allah (may Allah bless him and grant him peace) with news of them. He (peace be upon him) said, ‘If you wish, tell me; or, if you wish, I shall tell you.’ He replied, ‘Rather, tell me, O Messenger of Allah.’ The Messenger of Allah (may Allah bless him and grant him peace) related their entire story to him and described what had happened. He said, ‘By Him who sent you with the truth, you have not left a single detail of their story unmentioned. What happened to them was exactly as you have described.’