Sayyiduna Sa'd (may Allah be pleased with him) says that the Messenger of Allah (peace and blessings be upon him) left Sayyiduna Ali (may Allah be pleased with him) behind on the occasion of the expedition of Tabuk. Sayyiduna Ali (may Allah be pleased with him) said: O Messenger of Allah, are you leaving me among the women and children? He (peace and blessings be upon him) said: "Are you not pleased that you should have with me the position that Harun (peace be upon him) had with Musa (peace be upon him), except that there is no prophet after me?"
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 17893