What to say when thunder strikes

There is nothing authentic to the Prophet ﷺ in this regard. However, some of the people of knowledge, such as Sheikh Yahya hafidahullah, have said: "On the authority of 'Abdullāh ibn Zubair, that he would refrain from speaking when he heard thunder, saying: «سبحان الذي يسبح الرعد بحمده والملائكة من خيفته» "Glory be to the One whom thunder glorifies with His praise, and the angels out of fear of Him." Thereafter he would say, "This is indeed a severe warning for the people of the earth." This was narrated by Mālik in his Muwaṭṭa', and Bukhāri in his Book of Manners (الأدب المفرد), and it is authentic. So, there is no harm in saying this, as it is a Dhikr, and especially since there is nothing contradicting or negating it." ... This Dhikr is supported by the Qur'ān [Sūrah Ar-Ra'd - 13] and it (this Āyah) is evidence for it."