In the valley of Mandara Hill, Hiraṇyakaśipu began performing his austerities by standing with his toes on the ground, keeping his arms upward and looking toward the sky. This position was extremely difficult, but he accepted it as a means to attain perfection.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
स तेपे मन्दरद्रोण्यां तपः परमदारुणम् । ऊर्ध्वबाहुर्नभोदृष्टिः पादाङ्गुष्ठाश्रितावनिः
sa tepe mandaradroṇyāṃ tapaḥ paramadāruṇam | ūrdhvabāhurnabhodṛṣṭiḥ pādāṅguṣṭhāśritāvaniḥ
SB 7.3.2