The dua for protection from the evil eye is: 'A'udhu bikalimatillahit-tammati min kulli shaytanin wa hammah, wa min kulli 'aynin lammah' — I seek refuge in the perfect words of Allah from every devil and venomous creature, and from every evil eye. The Prophet used to seek this protection for Hasan and Husayn.
أُعِيذُكُمَا بِكَلِمَاتِ اللَّهِ التَّامَّةِ مِنْ كُلِّ شَيْطَانٍ وَهَامَّةٍ، وَمِنْ كُلِّ عَيْنٍ لَامَّةٍ
U'idhukuma bikalimatillahit-tammati min kulli shaytanin wa hammah, wa min kulli 'aynin lammah.
Meaning: I seek refuge for you both in the perfect words of Allah from every devil and venomous creature, and from every harmful evil eye.
Source: Sahih al-Bukhari #3371This dua has a chain going back to Prophet Ibrahim (peace be upon him), who used it to protect Isma'il and Ishaq. The Prophet (peace be upon him) continued this practice with his own grandsons. The evil eye is real — the Prophet confirmed it saying: 'The evil eye is real, and if anything could outstrip the divine decree, the evil eye would outstrip it.' (Sahih Muslim 2188)
أُعِيذُكُمَا بِكَلِمَاتِ اللَّهِ التَّامَّةِ مِنْ كُلِّ شَيْطَانٍ وَهَامَّةٍ، وَمِنْ كُلِّ عَيْنٍ لَامَّةٍ
The Prophet (peace be upon him) used to seek protection for Hasan and Husayn, saying: 'U'idhukuma bikalimatillahit-tammati min kulli shaytanin wa hammah, wa min kulli 'aynin lammah.' And he would say: 'Your forefather Ibrahim used to seek protection with these words for Isma'il and Ishaq.'