Surah Maryam 19:3

إِذْ نَادَىٰ رَبَّهُۥ نِدَآءً خَفِيًّا

When he called to his Lord a private call [i.e., supplication].

Translation: Saheeh International

Tafsir

when idh is semantically connected to rahma ‘mercy’ he called out to his Lord a call entailing a supplication in secret in the middle of the night because this time of the night invites a faster response to one’s supplication.

Excerpt from Tafsir al-Jalalayn, commentary on 19:3, via Quran.com. The reader has the full commentary and 9 more tafsirs.