Shiva Purana
गच्छतस्तिष्ठतो वापि स्वेच्छया कर्म कुर्वतः ॥ अशुचेर्वा शुचेर्वापि मन्त्रो ऽयन्न च निष्फलः ॥ ६३ ॥
This mantra is not ineffective when repeated by a person whether walking or standing or doing any other work or whether he is pure or impure.
english translation
gacchatastiSThato vApi svecchayA karma kurvataH ॥ azucervA zucervApi mantro 'yanna ca niSphalaH ॥ 63 ॥
hk transliteration by Sanscript