Rigvidhana 2.2.7

2.2.7

pūrṇām titīrṣuḥ saritam ramadhvam iti saṃsmaret./
o sv ity ṛcam apām madhye japed yo vai nadīm taran.//

Wishing to cross a full river, he should contemplate, "Ramadhvam" ("Rest ye"). / When crossing a river, one should recite in the midst of the waters the verse, "O ṣu" ("O come").