Musnad Ahmad 33:504

Hassan narrated to us, Ibn Lahi'ah narrated to us, Abu al-Zubayr narrated to us, from Jabir, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever says when the caller calls: 'O Allah, Lord of this perfect call, and the established prayer, send blessings upon Muhammad, and be pleased with him, a pleasure after which there is no displeasure,' Allah will answer his supplication for him."