Shiva Purana
अन्येषां च यथा शक्तिमध्याह्ने च शतं जपेत् । सायं द्विदशकं ज्ञेयं शिखाष्टकसमन्वितम् ३९ ।
Others shall repeat as many times as they can. In the midday Gāyatrī shall be repeated a hundred times; in the evening at least twenty times along with Śikhāṣṭaka ॥ A set of eight as the tuft i.e. eight times more than stipulated.॥
english translation
anyeSAM ca yathA zaktimadhyAhne ca zataM japet । sAyaM dvidazakaM jJeyaM zikhASTakasamanvitam 39 ।
hk transliteration by Sanscript