Sayyiduna Abu Rafi' (may Allah be pleased with him) narrates: The Prophet (peace and blessings be upon him) sent Sayyiduna Ali (may Allah be pleased with him) to Yemen and tied a banner for him. When he had set off, he (peace and blessings be upon him) said: "O Abu Rafi'! Go after him, and do not call out to him from behind; let him stop and not turn around until I myself come to him." Then he (peace and blessings be upon him) came to him and advised him about several things, and said: "O Ali! That Allah guides a single man through your hands is better for you than everything upon which the sun rises."
Hadith Referenceالمعجم الكبير للطبراني / باب الألف / 990