Riyad as-Salihin

Qailah bint Makhramah (May Allah be pleased with her) reported: I saw the Prophet (ﷺ) seated with his arms enfolding his legs; and when I saw him in such a state of humble guise I trembled with fear due to the awe (he showed in that posture).

[Abu Dawud and At-Tirmidhi].