Hi Paribajjaka,
The compound is probably "sīti-bhūtaṃ".
And the sources about Nāḷāgiri are listed in the article:
http://www.aimwell.org/DPPN/naalaagiri.htm
Here's a story from Culavagga (note that "sīti-bhūtaṃ" isn't used here):
Nāḷāgiripesanaṃ
342. Tena kho pana samayena rājagahe nāḷāgiri nāma hatthī caṇḍo hoti, manussaghātako. Atha kho devadatto rājagahaṃ pavisitvā hatthisālaṃ gantvā hatthibhaṇḍe etadavoca – ‘‘mayaṃ kho, bhaṇe, rājañātakā nāma paṭibalā nīcaṭṭhāniyaṃ uccaṭṭhāne ṭhapetuṃ, bhattampi vetanampi vaḍḍhāpetuṃ. Tena hi, bhaṇe, yadā samaṇo gotamo imaṃ racchaṃ paṭipanno hoti, tadā imaṃ nāḷāgiriṃ hatthiṃ muñcetvā imaṃ racchaṃ paṭipādethā’’ti. ‘‘Evaṃ bhante’’ti kho te hatthibhaṇḍā devadattassa paccassosuṃ. Atha kho bhagavā pubbaṇhasamayaṃ nivāsetvā pattacīvaramādāya sambahulehi bhikkhūhi saddhiṃ rājagahaṃ piṇḍāya pāvisi. Atha kho bhagavā taṃ racchaṃ paṭipajji. Addasāsuṃ kho te hatthibhaṇḍā bhagavantaṃ taṃ racchaṃ paṭipannaṃ. Disvāna nāḷāgiriṃ hatthiṃ muñcitvā taṃ racchaṃ paṭipādesuṃ. Addasā kho nāḷāgiri hatthī bhagavantaṃ dūratova āgacchantaṃ. Disvāna soṇḍaṃ ussāpetvā pahaṭṭhakaṇṇavālo yena bhagavā tena abhidhāvi. Addasāsuṃ kho te bhikkhū nāḷāgiriṃ hatthiṃ dūratova āgacchantaṃ. Disvāna bhagavantaṃ etadavocuṃ – ‘‘ayaṃ, bhante, nāḷāgiri hatthī caṇḍo manussaghātako imaṃ racchaṃ paṭipanno. Paṭikkamatu, bhante, bhagavā; paṭikkamatu sugato’’ti. ‘‘Āgacchatha, bhikkhave, mā bhāyittha. Aṭṭhānametaṃ, bhikkhave, anavakāso, yaṃ parūpakkamena tathāgataṃ jīvitā voropeyya. Anupakkamena, bhikkhave, tathāgatā parinibbāyantī’’ti. Dutiyampi kho te bhikkhū…pe… tatiyampi kho te bhikkhū bhagavantaṃ etadavocuṃ – ‘‘ayaṃ, bhante, nāḷāgiri hatthī caṇḍo manussaghātako imaṃ racchaṃ paṭipanno. Paṭikkamatu, bhante, bhagavā; paṭikkamatu sugato’’ti. ‘‘Āgacchatha, bhikkhave, mā bhāyittha. Aṭṭhānametaṃ, bhikkhave, anavakāso, yaṃ parūpakkamena tathāgataṃ jīvitā voropeyya. Anupakkamena, bhikkhave, tathāgatā parinibbāyantī’’ti.
Tena kho pana samayena manussā pāsādesupi hammiyesupi chadanesupi āruḷhā acchanti. Tattha ye te manussā assaddhā appasannā dubbuddhino, te evamāhaṃsu – ‘‘abhirūpo vata, bho [abhirūpo vata bho gotamo (syā. kaṃ.)], mahāsamaṇo nāgena viheṭhīyissatī’’ti. Ye pana te manussā saddhā pasannā paṇḍitā byattā buddhimanto, te evamāhaṃsu – ‘‘nacirassaṃ vata, bho, nāgo nāgena saṅgāmessatī’’ti. Atha kho bhagavā nāḷāgiriṃ hatthiṃ mettena cittena phari. Atha kho nāḷāgiri hatthī bhagavato [bhagavatā (sī.)] mettena cittena phuṭṭho [phuṭo (ka.)] soṇḍaṃ oropetvā yena bhagavā tenupasaṅkami, upasaṅkamitvā bhagavato purato aṭṭhāsi. Atha kho bhagavā dakkhiṇena hatthena nāḷāgirissa hatthissa kumbhaṃ parāmasanto nāḷāgiriṃ hatthiṃ imāhi gāthāhi ajjhabhāsi –
‘‘Mā kuñjara nāgamāsado, dukkhañhi kuñjara nāgamāsado;
Na hi nāgahatassa kuñjara sugati, hoti ito paraṃ yato.
‘‘Mā ca mado mā ca pamādo, na hi pamattā sugatiṃ vajanti te;
Tvaññeva tathā karissasi, yena tvaṃ sugatiṃ gamissasī’’ti.
Atha kho nāḷāgiri hatthī soṇḍāya bhagavato pādapaṃsūni gahetvā uparimuddhani ākiritvā paṭikuṭiyova [paṭikuṭito paṭisakki (sī. syā.)] osakki, yāva bhagavantaṃ addakkhi. Atha kho nāḷāgiri hatthī hatthisālaṃ gantvā sake ṭhāne aṭṭhāsi. Tathā danto ca pana nāḷāgiri hatthī ahosi. Tena kho pana samayena manussā imaṃ gāthaṃ gāyanti –
[ma. ni. 2.352; theragā. 878] ‘‘Daṇḍeneke damayanti, aṅkusehi kasāhi ca;
Adaṇḍena asatthena, nāgo danto mahesinā’’ti.
Manussā ujjhāyanti khiyyanti vipācenti – ‘‘yāva pāpo ayaṃ devadatto, alakkhiko, yatra hi nāma samaṇassa gotamassa evaṃmahiddhikassa evaṃ mahānubhāvassa vadhāya parakkamissatī’’ti. Devadattassa lābhasakkāro parihāyi. Bhagavato ca lābhasakkāro abhivaḍḍhi.
English translation:
http://www.sacred-texts.com/bud/sbe20/sbe20076.htm
Metta, Dmytro