Sayyiduna Anas (may Allah be pleased with him) says that the Prophet (peace and blessings be upon him) said: "Whoever helps his brother in his absence, Allah will help him in this world and the Hereafter."
It has also been narrated from al-Hasan, from Imran bin Husayn as mawquf (his own statement), and also as marfu'; the mawquf is more correct, and Allah knows best.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قتال أهل البغي / 16685