I think it is quoted in the Pāñcarātrarakṣā and prescribed in the Dakṣa-smṛti. According to S. M. Srinivasa Chari, regarding Pañcakāla-prakriyā:
The Pañcakāla-prakriyā of the Pāñcarātra ystem is formulated from out of the religious duties prescribed by the Smṛti texts and fitted into the daily life of a Vaiṣṇava to be observed at five specific times of the day as a form of dedication of one’s life to the service of Viṣṇu...
Then he quotes in a footnote a Sanskrit reference regarding Pañcakāla-prakriyā the quoted from the Pāñcarātra-rakṣa by Vedānta Deśika :
(Pāñcarātra-rakṣa p. 108)
kālapañcaka-vibhāgena abhigamanopādāna ījyāsvādyāyayogarūpa bhagavatsevanaṃ sva-varṇāśrama-jāti guṇa-nimittādi-niyatadharma-sacivaṃ sva-varṇāśrama-jāti guṇa-nimittādi-niyatadharma-sacivaṃ bhagavad-dharma-niṣṭhānāṃ sarveṣāṃ samānam.
Further down the text, he explains the fifth pañcakāla called yoga:
The fifth and final duty comes after completing the evening prayers and dinner before one goes to rest. This part of the day is called yoga. What is implied by yoga is that one should contemplate on God until he actually goes to sleep. At that time the individual should bring himself to feel that his self is resting in God.
According to yet another footnote, he quotes from the Dakṣa-smṛti
Dakṣa-smṛti (quoted by Vedānta Deśika regarding in Pāñcarātra-rakṣa p. 118)
sarvopādhi-vinirmuktaṃ kṣetrajñaṃ brahmaṇi nyaset; etat dyānaṃ ca yogaśca...
According to the English introduction, the “Pāñcarātrarakṣā is the third work of the Pāñcarātra-āgama”.