Mujaahid, may Allah have mercy on him, said: “When one of them stood in prayer, he would be too fearful of his Lord to allow his eyes to be drawn to anything, or to turn aside or to fidget by playing with pebbles or anything else or to think of any worldly matter, unless he forgot, during prayer.”
(Ta’zeem Qadr al-Salaah, 1/188)
Shaykh al-Islam Ibn Taymiyah, may Allah have mercy on him, when he started to pray, used to tremble so much that he would lean right and left.
(Al-Kawaakib al-Durriyah fi Manaaqib al-Mujtahid Ibn Taymiyah, by Mar’i al-Karami, p. 83).