When he called his Lord (with) a call in secret.He only made his supplication secretly because it is more beloved to Allah.This is similar to what Qatadah said concerning this Ayah, إِذْ نَادَى رَبَّهُ نِدَاء خَفِيًّا(When he called out his Lord (with) a call in secret),"Verily, Allah knows the pious heart and he hears the hidden voice.