Sayyiduna Abu Hurayrah (may Allah be pleased with him) reports this saying of the Prophet (peace and blessings be upon him): "Part of the completeness of a servant's faith is that he makes the exception (that is, says in sha' Allah) in everything he says."
Imam al-Tabarani recorded it in al-Mu'jam al-Awsat; its chain contains a narrator, Abdullah bin Sa'id bin Abi Sa'id, who is da'if (weak).