The supplication of a Muslim for his brother in his absence

Shahr ibn Hawshab reported: I asked Umm Salamah (may Allah be pleased with her): "O Mother of the Believers, what was the supplication that the Messenger of Allah (may Allah's peace and blessings be upon him) used to say most frequently when he was with you?" She said: "The supplication he said most frequently was: 'O Turner of the hearts, keep my heart firm upon Your religion!'" [At-Tirmidhi].