Skandapurana 5.266.18

5.266.18

tanmadhye suśubhaṃ kāṣṭhaṃ staṃbhajātyaṃ dṛḍhaṃ nyaset..
candanaḥ khadiro vātha bilvovā'śvattha eva vā.. 18..

Between them he should place a very auspicious, strong crossbeam made of timber suitable for pillars: / either sandalwood, acacia, bilva, or aśvattha;