Shiva Purana

siva.sh/siva-purana/vishweshwara-samhita/13/29

सायं मुहूर्तादर्वाक्तु कृता संध्या वृथा भवेत् । अकालात्काल इत्युक्तो दिनेऽतीते यथाक्रमम् २९ ।

sanskrit

Sandhyā prayer performed before the prescribed time is ineffective. Hence Sandhyā shall be performed at the prescribed time. The expiatory rite for the omission of Sandhyā prayer for a day is the repetition of Gāyatrī a hundred times more than the usual number of times for ten days.

english translation

sAyaM muhUrtAdarvAktu kRtA saMdhyA vRthA bhavet | akAlAtkAla ityukto dine'tIte yathAkramam 29 |

hk transliteration by Sanscript