Śatapatha Brāhmaṇa ("of a hundred paths") 6.6.2.4

6.6.2.4

yadvevodaṅ prāṅ tiṣṭhan | etasyāṃ ha diśi svargasya lokasya dvāraṃ tasmādudaṅ prāṅ tiṣṭhannāhutīrjuhotyudaṅ prāṅ tiṣṭhandakṣiṇā nayati dvāraiva tatsvargasya lokasya vittam prapādayati

And, again, why standing towards north-east,--in that quarter is the gate of the world of heaven, hence it is standing with his face towards northeast that one offers libations, and standing towards north-east that one leads up the dakshinâs: it is by the gate that he thus makes him enter into the world of heaven.