Bhagavatapurana 4.8.53

4.8.53

japaś ca paramo guhyaḥ śrūyatāṃ me nṛpātmaja /
yaṃ sapta-rātraṃ prapaṭhan pumān paśyati khecarān //

53\. There is a most secret *mantra* which should be mentally repeated over and over while contemplating the Lord and His attributes; let it be heard and known from me; any one who repeats it seven nights sees the gods moving through the sky.