Sayyiduna Ali bin Abi Talib (may Allah be pleased with him) narrates: We stopped at a place and fleas troubled us, so we reviled them. The Prophet (peace and blessings be upon him) said: "Do not revile them, for it is a good creature; it has woken you for the remembrance of Allah."
Al-Tabarani recorded it in al-Mu'jam al-Awsat. Its chain contains a narrator, Sa'd bin Turayq, and he is matruk (abandoned).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الأدب / 13043