Shiva Purana
शनैश्चरं तथा राहुं केतुं धूम्रं भयंकरम् ॥ समंततो यजेदेतांस्तृतीयावरणे क्रमात् ॥ ३३ ॥
Saturn, Rahu, Ketu and Dhumra are the terrible signs. One should worship these on all sides in the third covering in order.
english translation
zanaizcaraM tathA rAhuM ketuM dhUmraM bhayaMkaram ॥ samaMtato yajedetAMstRtIyAvaraNe kramAt ॥ 33 ॥
hk transliteration by Sanscript