It is narrated from Sayyiduna Ibn Amr, may Allah be pleased with him, that the Noble Prophet, peace and blessings be upon him, said: "Whoever gives allegiance to an Imam and gives him the pledge of his hand and the sincerity of his heart, then he should obey him as much as he can."
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 6815