Svadhyaya or Brahma Yajna is a Vedic ritual and involves the recitation of Vedic hymns. The earliest reference to this is found in Sathapatha Brahmana XI 5 – 6, 3 – 8.
Manusmriti (2.28) maintains that a body is made fit for realizing Brahman by Vedic studies, by observing vows, offering oblations, performing rituals, procreation of children and practicing the five mahayajnas (five sacred rituals for gods, seers, ancestors, fellowman and creatures of God) of which Brahmayajna is a part.
Taittirya Aranyaka II 1.1 prescribes the mantras to be chanted in the performance of Brahmayajna. It also specifies the place where it is to be performed.
Brahma Yajna is a solemn obligation of a householder to be discharged to the seers of Vedas. The householder should bathe in the east or north of his village, sip water on a clean spot and wear his sacred thread upon his left shoulder. His garments should not be wet. He should then spread a large quantity of durva grass (panic grass) pointing towards the east, and sit on it facing east by putting his right in his left hand in his lap. Flexibility in posture is allowed. Either looking at the horizon or closing his eyes, one should perform svadhyaya. The vyahriti (the words bhur bhuvah swah) preceded by the syllable om are to be pronounced first. The he should repeat Savitri, first, hemistich by hemistich, then line by line and finally the whole verse at a stretch.
If one is unable to perform it in house or village, one may perform Brahma Yajna by intoning Vedic hymns in his mind anytime of the day or night. If the circumstances do not permit one to sit as prescribed due to bodily disabilities, then one may perform Brahma Yajna while standing or even lying down. This is also called ahuta.