Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) says: We were sitting with the Messenger of Allah (peace and blessings be upon him) when Hasan and Husayn (may Allah be pleased with them both) passed by him (peace and blessings be upon him), while they were still small children. He (peace and blessings be upon him) said: "Bring my two sons, so that I may seek protection for them with the words with which Ibrahim (peace be upon him) sought protection for his two sons Isma'il and Ishaq (peace be upon them both)." Then he said: "'U'idhukuma bi kalimatillahit-tammah min kulli 'aynin lammah, wa min kulli shaytanin wa hammah' (I seek protection for you both in the perfect words of Allah from every evil eye, and from every devil and venomous creature)."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9948